Cellular wireless communications systems are known, wherein a geographical area is divided into cells, and each cell includes a base station (BS) for communicating with subscriber units (SUs) (also called remote terminals, mobile units, mobile stations, subscriber stations, or remote users) within the cell. We have previously described cellular systems that have BSs that include an array of antenna elements and spatial processing means. When used as receivers, the array of antenna elements introduce multiple versions of each signal, each of these versions comprising the composite of all the co-channel signals together with interference and noise. With multiple antennas, the relationship in both amplitude and phase of a signal of interest from a particular remote user to the interfering co-channel signals (i.e., signals from other remote users) will be different in each of the antenna signals due, for example, to geometric considerations, both because the antennas are separated by some distance, and, in some cases, because the different remote users also are separated. Using such an antenna array, spatial processing by weighting the received signals in amplitude and phase by different weights provides many advantages, including the possibility of spatial division multiple access (SDMA) techniques, in which the same "conventional channel" (i.e., the same frequency channel in a frequency division multiple access (FDMA) system, timeslot in a time division multiple access (TDMA) system, code in a code division multiple access (CDMA) system, or timeslot and frequency in a TDMA/FDMA system) may be assigned to more than one subscriber unit.
Some examples of a cellular system are digital systems which use variants of the Personal Handy Phone System (PHS) protocol defined by the Association of Radio Industries and Businesses (ARIB) Preliminary Standard, RCR STD-28 (Version 2) December 1995, and digital systems that use the Global System for Mobile communications (GSM) protocol, including the original version, 1.8 GHz version called DCS-1800, and the North American 1.9 GHz personal communications system (PCS) version called PCS-1900.
When a signal is sent from a remote unit to a base station (i.e., communication is in the uplink), the base station having a receiving antenna array (usually, and not necessarily the same antenna array as for transmission), the signals received at each element of the receiving array are each weighted, typically after downconversion (i.e., in baseband), in amplitude and phase by a receive weight (also called spatial demultiplexing weight), this processing called spatial demultiplexing, or spatial processing, all the receive weights determining a complex valued receive weight vector which is dependent on the receive spatial signature of the remote user transmitting to the base station. The receive spatial signature characterizes how the base station array receives signals from a particular subscriber unit in the absence of any interference. This invention is described for uplink communications in a cellular system, although the techniques certainly are applicable to the design of any receiver for any digitally modulated signal where it is desired to reduce the effects of frequency offset.
In systems that use antenna arrays, the weighting of the baseband signals either in the uplink from each antenna element in an array of antennas, or in the downlink to each antenna element is called spatial processing herein. Spatial processing is useful even when no more than one subscriber unit is assigned to any conventional channel. Thus, the term SDMA shall be used herein to include both the true spatial multiplexing case of having more than one user per conventional channel, and the use of spatial processing with only one user per conventional channel. The term channel shall refer to a communications link between a base station and a single remote user, so that the term SDMA covers both a single channel per conventional channel, and more than one channel per conventional channel. The multiple channels within a conventional channel are called spatial channels. For a description of SDMA systems that can work with more than one spatial channel per conventional channel, see, for example, co-owned U.S. Pat. Nos. 5,515,378 (issued May 7, 1996) and 5,642,353 (issued Jun. 24, 1997) entitled SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEMS, Roy, III, et al., inventors, both incorporated herein by reference; and co-owned U.S. Pat. No. 5,592,490 (issued Jan. 7, 1997) entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, Barratt, et al., inventors. The Parent Patent describes demodulation in a SDMA system that has only one spatial channel per conventional channel.
SDMA systems use spatial processing as the backbone to improve system capacity and signal quality. In the Parent patent, we described generating a reference signal from the received antenna signals, and how the reference signal can then be used to determine the spatial demultiplexing weights. In such a system, the performance of the spatial processor depends on many factors, including:
The input signal-to-noise ratio (SNR); PA1 The number of interferers or carrier-to-interference ratio (CIR); PA1 The spatial correlation between the users; and PA1 The quality of the reference signal.
Each of these will now be briefly explained. The input SNR at the antenna elements is determined by the transmitted power of the subscriber unit, the antenna gains, the path losses, and other RF effects.
The input CIR is determined by the transmitted power of the subscriber unit, and the powers of the other users and interferers occupying the same conventional channel (e.g., same frequency band) or emitting energy in that channel.
The reference signal is the replica of the transmitted signal that is generated at the receiver to train the demultiplexing weights for the signals received by the antenna array elements. The quality of the reference signal determines the nulling ability of the array. In the uplink, the improvement in the nulling ability of the array results in an increase in the output SINR. Therefore if the quality of the reference signal is improved, the BER performance in the uplink is improved. Improving the quality of reference signal generation and demodulation is the subject of this invention.
The receive (copy) weights may be determined from samples of the input signal and from the reference signal.
Thus there clearly is a need for improved demodulation and reference signal generation methods and systems for use in communication systems that include an antenna array and spatial processing.
The Parent Patent described the use of a demodulator/reference signal generator that tracked the frequency offset from sample to sample by relaxing the phase expected from the modulation scheme back towards the actual phase of the input signal. The present invention extends these methods.